This invention relates generally to the field of disc drive storage devices, and more particularly, but not by way of limitation, to improving seek performance of a disc drive servo system through adaptive servo gain adjustment.
Disc drives are digital data storage devices which enable users of computer systems to store and retrieve large amounts of data in a fast and efficient manner. Disc drives of the present generation have data storage capacities in excess of several gigabytes (GB) and can transfer data at sustained rates of several megabytes (MB) per second.
A typical disc drive is provided with a plurality of magnetic recording discs which are mounted to a rotatable hub of a spindle motor for rotation at a constant, high speed. An array of read/write heads is disposed adjacent surfaces of the discs to transfer data between the discs and a host computer. The heads are radially positioned over the discs by a rotary actuator and a closed loop, digital servo system, and are caused to fly proximate the surfaces of the discs upon air bearings established by air currents set up by the high speed rotation of the discs.
A plurality of nominally concentric tracks are defined on each disc surface, with disc drives of the present generation having track densities in excess of 4,000 tracks per centimeter (10,000 tracks per inch). A preamp and driver circuit generates write currents that are used by the head to selectively magnetize the tracks during a data write operation and amplifies read signals detected by the head from the selective magnetization of the tracks during a data read operation. A read/write channel and interface circuit are operably connected to the preamp and driver circuit to transfer the data between the discs and the host computer.
The servo system operates in two primary modes: seeking and track following. During a seek, a selected head is moved from an initial track to a destination track on the corresponding disc surface using a velocity-controlled approach. The servo system initially determines a velocity profile indicative of the velocity trajectory the head should take based upon the length of the seek (in terms of tracks to go to the destination track). The servo system next applies current to the actuator coil to first accelerate and then decelerate the head toward the destination track in accordance with the velocity profile.
During the seek, the servo system sequentially measures (estimates) the actual velocity of the head and adjusts the current in relation to velocity error (i.e., the difference between the actual velocity and the target velocity as set forth by the velocity profile). As the head approaches the destination track, the servo system initiates a settle mode to bring the head to rest over the destination track within a selected settle threshold as a percentage of the track width, such as xc2x110% of track center. Thereafter, the servo system enters the track following mode wherein the head is nominally maintained over the center of the destination track until the next seek is performed.
As will be recognized, a disc drive is primarily utilized to transfer data between the tracks of the discs and the host computer. Such data transfer operations usually cannot occur during a seek, but rather require the drive to be in track following mode. Hence, to maximize disc drive data transfer rate capabilities, high performance disc drives are designed to achieve minimum average seek times, with such drives of the present generation having nominal seek times on the order of eight milliseconds (msec).
Achieving consistently minimum seek times is complicated by the fact that disc drive settle performance can be adversely affected by velocity errors present as the servo system transitions from seek to settle mode. Such an initial velocity variation can cause large overshoot or undershoot of the head relative to the destination track, undesirably extending the time required to settle the head onto the destination track. This velocity variation is inherent in modern disc drive designs because the acceleration constant of the VCM changes with temperature and relative position of the coil. These and other such factors tend to introduce velocity errors during seeks, undesirably extending settling times.
Accordingly, there is a continual need in the art for improvements whereby settling characteristics of a disc drive can be adaptively optimized in the presence of parametric variations which tend to introduce significant levels of head velocity errors.
The present invention provides an apparatus and method for improving servo performance of a disc drive through adaptive servo gain adjustment.
As exemplified in preferred embodiments, a disc drive is provided with a head adjacent a rotatable disc, and a servo circuit which controllably positions the head adjacent tracks defined on the disc.
During a seek operation in which the servo circuit moves the head from an initial track to a destination track in relation to a velocity profile indicative of a desired velocity trajectory of the head, the servo circuit accumulates a velocity error as a sum of a series of velocity errors each obtained as a difference between an actual velocity of the head and a corresponding desired velocity of the head as set forth by the velocity profile over a selected radial distance of the disc.
The servo circuit additionally measures an elapsed time for the head to pass from a point at a selected distance from the destination track until the head is settled onto the destination track. Thereafter, the servo circuit adjusts a gain of the servo circuit used to control the disc drive during a subsequent seek operation in relation to the accumulated velocity error and the measured elapsed time. In an alternative preferred embodiment, the gain is adjusted solely in relation to the measured elapsed time without regard to the accumulated velocity error.
The servo circuit preferably comprises an observer which provides estimates of head position and velocity during the seek operation, a gain block operably coupled as an input to the observer which applies the gain to a current correction command signal to improve the estimates of head position and velocity provided by the observer and a gain adjust block, operably coupled to the observer and the gain block, which provides the adjusted gain to the observer. The gain is utilized to maintain the observer response nominally that of the disc drive.
Improved servo performance is thus obtained through the convergence of the gain over a series of successive seeks, compensating for factors (such as changes in temperature) which tend to adversely affect servo performance and lengthen settling times due to the introduction of velocity errors as the head is settled onto the destination track.
These and various other features as well as advantages which characterize the present invention will be apparent from a reading of the following detailed description and a review of the associated drawings.